This invention relates generally to lasers and more particularly to a mode-coupled laser system wherein the laser beam is mode-coupled by means of a pulsed switching signal having spaced short pulses.
For clarification, reference herein to the pulsed output beam and switching signal having "spaced short pulses" indicates the width of the spaced pulses is considerably shorter than the cavity round trip time and is not a sinusoidal modulation signal.
As is well known, the period T of the output signal from a standing-wave laser cavity of length L is, ##EQU1##
A pulsed periodic laser output signal has a set of frequency sidebands or axial modes which are to some degree coupled in phase. It is desirable to produce mode-coupled laser signals or beams having short pulsewidths for various scientific and industrial uses as it provides improved time resolution higher peak intensity.
Active and passive mode coupling of pulsed laser output signals are well known. Active mode coupling has usually been achieved by driving a modulator located in the laser cavity with a sinusoidal electrical modulation signal having a frequency which is closely tuned to the round-trip frequency of the laser cavity to cause the formation of pulses. Sinusoidal modulation produces relatively long laser pulses because of the inherently long switching signal. Theoretical analysis based on gaussian-pulse approximation shows that the laser pulsewidth is directly proportional to the square root of the switcing pulsewidth and inversely proportional to the fourth root of the switching pulse amplitude. Passive mode coupling has been achieved by locating a saturable absorber element in the laser cavity which has an absorption depending on the optical intensity. This has produced shorter pulsewidths than those achievable by active mode coupling because of the shorter pulsewidth switching signal provided by the non linear saturable absorber. While passive mode coupling achieves shorter pulsewidths, it has the disadvantages that it does not allow any external synchronization, reduces efficiency of the laser, and is chemically degradable.
Active mode locking is known using feedback of the fundamental cavity frequency from the output laser beam to drive the modulator via an amplifier to ensure frequency locking of the modulation frequency with the round-trip frequency of the laser cavity Such an arrangement is described by T. S. Kinsel et al in "A Stabilized Mode-Locked Nd:YAG Laser Source", IEEE J. Quantum Electronics, June 1969. However, this method still uses a sinusoidal modulation signal and does not produce a shorter output pulsed signal.